virtual_sdcard: prepare for pause_resume implementation

- Add is_active() member function to return the active printing status of the virtual sdcard
- M27 now reports current file position when file has been opened. See http://marlinfw.org/docs/gcode/M027.html

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

klippy/extras/virtual_sdcard.py

@@ -57,6 +57,8 @@ class VirtualSD:
         if self.work_timer is not None and self.file_size:
             progress = float(self.file_position) / self.file_size
         return {'progress': progress}
+    def is_active(self):
+        return self.work_timer is not None
     # G-Code commands
     def cmd_error(self, params):
         raise self.gcode.error("SD write not supported")
@@ -123,7 +125,7 @@ class VirtualSD:
         self.file_position = pos
     def cmd_M27(self, params):
         # Report SD print status
-        if self.current_file is None or self.work_timer is None:
+        if self.current_file is None:
             self.gcode.respond("Not SD printing.")
             return
         self.gcode.respond("SD printing byte %d/%d" % (